Allelopathy via caulerpin

Lore & Background

On many Indo-Pacific and Caribbean reefs, the boundary between a healthy coral colony and a creeping mat of Caulerpa taxufolia is not a sharp line but a chemical front. Caulerpin, released continuously from the algal stromata and stolons, diffuses through the thin boundary layer that coats the coral surface. Once it contacts the coral's mucus layer and underlying tissue, it interferes with the aragonite-deposition machinery of the calcifying epithelium. The visible result is a halo of bleached, then necrotic, tissue at the algal–coral interface, while the alga itself remains metabolically active and continues to expand.

The ecological significance extends beyond a single pairwise interaction. By suppressing coral recruitment and accelerating the loss of existing colonies, caulerpin-mediated allelopathy can tip a reef from coral-dominated to algal-dominated states, particularly where overfishing of herbivorous fish and urchins removes the grazing pressure that would otherwise keep Caulerpa in check. In this sense, the compound acts as a positive-feedback agent in reef phase-shift dynamics: more algal cover releases more caulerpin, which kills more coral, which opens more substrate for further algal colonisation.

Caulerpin is not the only allelopathic compound in the Caulerpa–coral interaction; additional terpenoids and polyacetylenes have been reported from the same species. However, caulerpin remains the most studied and the one for which dose-dependent effects on coral calcification and tissue viability have been most clearly demonstrated in laboratory and mesocosm experiments.

Reader's Guide

Relationship Card: Caulerpa taxufolia × Scleractinian coral (e.g., Porites, Acropora)

Parties: (1) Caulerpa taxufolia, a free-living green macroalga spreading via stolons and stromata over hard substrate; (2) a scleractinian coral colony, polyp-covered and actively calcifying.

What each gains or loses: The alga gains substrate space and reduced competition for light and nutrients; the coral loses tissue, calcification rate, and ultimately colony area. The alga pays a metabolic cost in synthesising and exuding the sesquiterpene lactone. The coral pays the cost of tissue repair and, if the chemical front is sustained, mortality.

Mechanism: Chemical (waterborne allelopathy). Caulerpin diffuses from the algal surface through the diffusive boundary layer onto the coral mucus and epithelium, inhibiting aragonite deposition and triggering necrosis. No physical overgrowth or direct predation is required; the alga can remain millimetres away.

Net effect on local community: Coral cover declines, algal cover increases, habitat complexity drops, and the recruitment substrate for coral planulae shrinks. Herbivorous fish and urchins that would graze the alga become the critical counter-pressure; where they are depleted, the allelopathic feedback accelerates and the reef shifts toward a macroalgal-dominated state.

Did You Know?

• Caulerpin is a sesquiterpene lactone, placing it in the same broad terpenoid family as many plant defensive compounds, but its marine allelopathic role on coral reefs was only characterised in the early 1990s.
• The compound does not require physical contact with the coral; it acts across the millimetre-scale diffusive boundary layer, meaning the alga can suppress coral calcification from a short distance.
• Caulerpa taxufolia is a green alga (Chlorophyta), not a red or brown alga, yet its allelopathic chemicals are among the most studied in marine ecology.
• In reef phase-shift models, caulerpin-mediated allelopathy acts as a positive feedback loop: more algal cover releases more compound, which kills more coral, which opens more space for the alga.